Science —

Soot in soils may decrease carbon dioxide feedback

We may not know all we think we know about carbon in soils and its impact on …

Atmospheric CO2 is an important driver of the global climate system and, as such, understanding the global carbon cycle is important to predicting future climate change. While we have a good handle on the amount of CO2 we are emitting, the net ecosystem fluxes are less well quantified.

In general, the natural fluxes of carbon between the atmosphere and the land or ocean are far greater than our own emissions. Indeed, CO2 released by soils is about 10 times greater than anthropogenic emissions. Because the natural input and output fluxes are roughly balanced, our own emissions have a large impact on the net flux. However, if future climate change causes the natural CO2 fluxes to go out of balance, they could easily become more larger sources of CO2 than our own emissions. A recent paper in Nature Geoscience examines how climate change can cause potential changes in soil carbon emissions.

It is generally accepted that increasing temperatures lead to CO2 being transfered from soils to the atmosphere. However, Johannes Lehmann and colleagues suggest that current models may overestimate this increase. The study, released Sunday, showed that a model of soil carbon over-predicted the amount of labile, or unstable, carbon in soils. By looking at almost 1,900 measurements of soil carbon across Australia, they showed that, on average, 20 percent of soil carbon in these samples is in the form of black carbon, or soot.

Black carbon is important because it has a much longer residence time in soils—1,300 to 2,600 years—compared to more labile carbon pools, which range from 10-100 years. The authors estimate that this can lead some current models to over-predict the amount of climate change induced carbon release by 20-40%.

While this study is interesting because it highlights an area of the carbon cycle that needs further attention, it is far from definitive. For one thing, all of the soils they measured came from Australia, which is suspected of having undergone major biomass burning in the past, albeit longer ago than the mean residence time of soot.

In addition to a limited data set, the paper only tested a single model. Biogeochemical cycle models are plentiful, with the CENTURY model and the Biome-BGC model being two of the more commonly used ones. Biome-BGC is the basis for the biogeochemical cycle in the Community Land Model, which is part of a model used in IPCC projections. While these models may not do any better than the RothC model tested here, the authors make no mention of whether the other models handle soot better.